1. Field of the Invention
The invention relates to a titanium based, ceramic reinforced alloy ingot for use in producing medical implants. More particularly, the invention pertains to a ceramic reinforced alloy ingot comprising titanium, niobium and silicon. The alloy has both an α crystal phase and a β crystal phase. The ingot has an ultimate tensile strength of about 940 MPa or more, and a Young's modulus of about 150 GPa or less.
2. Description of the Related Art
There is great commercial interest in the production of biocompatible, medically suitable implants for surgically jointing bone and implanting teeth. Medical implants such as screws, pins, rods, bars, springs, coils, cables, staples, clips, plates and the like require materials with very high tensile strength and high cyclic fatigue life while also having a modulus of elasticity low enough to be compatible with bone. Common alloys include titanium, stainless steel and cobalt chrome alloys. Stainless steel and cobalt chrome alloys exhibit very high tensile strength, but both contain nickel and chromium which are known irritants to the body. In addition, these alloys have low ductility and a Young's modulus approaching five times that of bone. This high tensile strength and Young's modulus also makes it difficult to machine these components cost effectively using conventional techniques. Titanium and its alloys are especially popular choices for orthopedic bone screws and plates commonly used for spinal fixation. Titanium alloys for a variety of applications are known in the art and there are numerous literature references disclosing a wide range of elements which are used to provide alloys having desired characteristics, such as increased tensile strength and ductility. Generally, titanium and its alloys may exist in one or a mixture of two basic crystalline structures, namely the α phase, which is a hexagonal close-packed structure, and the β phase which is a body-centered cubic structure. The commercially pure grades of titanium alloys have low tensile strengths but show no signs of tissue irritation. These alloys are commonly used for orthopedic plates which are implanted externally to the bone structure and can therefore have a larger size. Ti6AlV4 alloys are commonly used for higher strength applications such as fixation screws or plates which must be contained in a small area. One known medically implantable alloy is disclosed in U.S. Pat. No. 6,752,882. It provides a biocompatible low modulus, high strength titanium-niobium alloy containing α phase as a major phase and consisting essentially of 10-30 wt % of Nb and the balance titanium. U.S. Pat. No. 5,954,724 relates to titanium alloys suitable for use for medical implants and devices having a high-strength, low-modulus, and high hardness with improved corrosion resistance due to the addition of hafnium and molybdenum, and which additionally allow for surface hardening of an implant made of this alloy. U.S. Pat. No. 7,892,369 provides a method for modifying the microstructure of titanium alloys for use in the manufacture of orthopedic prostheses. An orthopedic prosthesis is initially formed from a titanium alloy and subsequently subjected to a thermal treatment followed by rapid quenching. The microstructure of the titanium alloy in the prosthesis has improved resistance to fretting fatigue. U.S. Pat. No. 7,682,473 provides an implant prosthesis composed of a TiAlNb alloy having a modulus near that for bone to prevent stress shielding, and a tensile and compressive strength and fracture toughness equal to or greater than that of bone. A key problem with other alloys which use aluminum and vanadium is the suspected effect of Al and V when movement and fretting are involved. The release of Al and V into the blood stream could cause irritation for the patient in the long term. Another issue with certain grades of titanium is the so called “notch effect” during cyclic fatigue. Prepared and polished samples of certain titanium alloys have been shown to have fatigue strength near the ultimate tensile strength. However, when a notch is introduced to the sample, the fatigue strength can be lowered to 40% of the ultimate tensile strength. Since implantable devices must be laser marked with the appropriate tracking information, a notch situation always exists and care must be taken not to exceed the notch fatigue strength.
The problems associated with designing an implantable device are specifically, providing an alloy with high tensile strength, and a marginal Young's modulus that contains no known irritants which can be economically machined with conventional methods. The present invention addresses all these issues. The invention provides an alloy of titanium, niobium and silicon. Titanium and niobium alloys are known to form alloys with very low Young's modulus (50-80 GPa). A problem with these known alloys is that they do not have sufficient strength for the manufacture of orthopedic devices such as bone plates and fixation screws. This invention overcomes the limitations of conventional alloys by including within a solid solution of the metals, a glassy silicon ceramic which acts to absorb energy during crack propagation and retard dislocations during applied stress. The atomic percent of this glassy silicon ceramic is controlled as to still allow for a moderately low Young's modulus and good formability. The inventive alloy of primarily Ti with the addition of Nb and Si produces alloys which have a complex alpha/beta structure with an amount of glassy material. The resulting alloy has a higher strength then the titanium grades presently used in medical implants while retaining a comparable elastic modulus.